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TZSET(3)                   Linux Programmer's Manual                  TZSET(3)

       tzset, tzname, timezone, daylight - initialize time conversion information

       #include <time.h>

       void tzset (void);

       extern char *tzname[2];
       extern long timezone;
       extern int daylight;

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       tzset(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE
       tzname: _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE
       timezone: _SVID_SOURCE || _XOPEN_SOURCE
       daylight: _SVID_SOURCE || _XOPEN_SOURCE

       The  tzset() function initializes the tzname variable from the TZ environment variable.  This function is auto-
       matically called by the other time conversion functions that depend on the timezone.  In a System-V-like  envi-
       ronment it will also set the variables timezone (seconds West of GMT) and daylight (0 if this timezone does not
       have any daylight saving time rules, non-zero if there is a time during the  year  when  daylight  saving  time

       If the TZ variable does not appear in the environment, the tzname variable is initialized with the best approx-
       imation of local wall clock time, as specified by the tzfile(5)-format file localtime found in the system time-
       zone  directory (see below).  (One also often sees /etc/localtime used here, a symlink to the right file in the
       system timezone directory.)

       If the TZ variable does appear in the environment but its value is empty or its  value  cannot  be  interpreted
       using any of the formats specified below, Coordinated Universal Time (UTC) is used.

       The value of TZ can be one of three formats.  The first format is used when there is no daylight saving time in
       the local timezone:

              std offset

       The std string specifies the name of the timezone and must be three or more alphabetic characters.  The  offset
       string  immediately  follows  std and specifies the time value to be added to the local time to get Coordinated
       Universal Time (UTC).  The offset is positive if the local timezone is west of the Prime Meridian and  negative
       if it is east.  The hour must be between 0 and 24, and the minutes and seconds 0 and 59.

       The second format is used when there is daylight saving time:

              std offset dst [offset],start[/time],end[/time]

       There  are  no  spaces  in  the  specification.   The  initial std and offset specify the standard timezone, as
       described above.  The dst string and offset specify the name and offset for the corresponding  daylight  saving
       timezone.  If the offset is omitted, it default to one hour ahead of standard time.

       The  start  field  specifies  when  daylight  saving time goes into effect and the end field specifies when the
       change is made back to standard time.  These fields may have the following formats:

       Jn     This specifies the Julian day with n between 1 and 365.  February 29  is  never  counted  even  in  leap

       n      This specifies the Julian day with n between 0 and 365.  February 29 is counted in leap years.

       Mm.w.d This  specifies  day  d  (0 <= d <= 6) of week w (1 <= w <= 5) of month m (1 <= m <= 12).  Week 1 is the
              first week in which day d occurs and week 5 is the last week in which day d occurs.  Day 0 is a  Sunday.

       The  time  fields specify when, in the local time currently in effect, the change to the other time occurs.  If
       omitted, the default is 02:00:00.

       Here is an example for New Zealand, where the standard time (NZST) is 12 hours ahead of UTC, and daylight  sav-
       ing time (NZDT), 13 hours ahead of UTC, runs from the first Sunday in October to the third Sunday in March, and
       the changeovers happen at the default time of 02:00:00:


       The third format specifies that the timezone information should be read from a file:


       If the file specification filespec is omitted, the timezone information is read from the file localtime in  the
       system  timezone  directory,  which nowadays usually is /usr/share/zoneinfo.  This file is in tzfile(5) format.
       If filespec is given, it specifies another tzfile(5)-format file to read the  timezone  information  from.   If
       filespec does not begin with a '/', the file specification is relative to the system timezone directory.

       Here's an example, once more for New Zealand:


       The system timezone directory used depends on the (g)libc version.  Libc4 and libc5 use /usr/lib/zoneinfo, and,
       since libc-5.4.6, when this doesn't work, will try /usr/share/zoneinfo.  Glibc2 will use the environment  vari-
       able  TZDIR,  when  that exists.  Its default depends on how it was installed, but normally is /usr/share/zone-

       This timezone directory contains the files
       localtime      local timezone file
       posixrules     rules for POSIX-style TZ's

       Often /etc/localtime is a symlink to the file localtime or to the correct timezone file in the system  timezone

       SVr4, POSIX.1-2001, 4.3BSD.

       Note that the variable daylight does not indicate that daylight saving time applies right now.  It used to give
       the number of some algorithm (see the variable tz_dsttime in gettimeofday(2)).  It has been obsolete  for  many
       years but is required by SUSv2.

       4.3BSD  had  a  function  char *timezone(zone, dst) that returned the name of the timezone corresponding to its
       first argument (minutes West of GMT).  If the second argument was 0, the standard name was used, otherwise  the
       daylight saving time version.

       date(1), gettimeofday(2), time(2), ctime(3), getenv(3), tzfile(5)

       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at

                                  2008-08-29                          TZSET(3)